The present invention relates to fluxmeters using a superconducting quantum interference device, and more particularly to pickup coils (probe coils) and their cores in the fluxmeters suitable for detecting long-term embrittlement (concerning thermal aging) of a metal material such as ferrite stainless steel used under high temperature environments such as chemical plants and atomic power plants.
Japanese Patent Publication JP-A-62-277704 discloses a method of manufacturing superconducting sheet coils which exhibit a high critical current characteristic in a high magnetic field used for a nuclear fusion toroidal magnet, a particle accelerator magnet, a superconducting generator magnet, etc., but which do not exhibit such characteristics in a low magnetic field used, for example, in the diagnosis of the deterioration of ferrite stainless steel.
Japanese Patent Publication JP-A-61-28859 discloses a method of detecting the degree of long-term embrittlement damage of a ferrite stainless steel member used in an actual plant. In this method, a change in the quantity of ferrite in the actual plant Member after long term use under high-temperature conditions is magnetically measured using a ferrite scope.
In the Publication JP-A-61-28859, a change in the quantity of ferrite is measured using a ferrite scope, so that it is impossible to measure a change in the magnetic characteristic due to the deposition of .varies.' and G phases produced by the spinodal decomposition of the initial ferrite phase.
Japanese Patent Publication JP-A-62-277704 is concerned with superconductors of Nb.sub.3 Sn superconducting intermetallic compound, and discloses a method of manufacturing a superconducting sheet coil, comprising the steps of superimposing a substrate containing at least one of two or more metal elements constituting a superconducting intermetallic compound and a metal plate containing the remaining metal element, inserting therebetween an additive member containing a third element, for example, of Ti or Ta, to improve the critical current density in the high magnetic field, and forming a circuit using a heating beam such as a laser beam or an electron beam. According to this method, the superconducting circuit can be produced by adding the third element without alloying the substrate with the metal plate, so that superconducting coils can be manufactured which exhibit an excellent critical current characteristic in a high magnetic field. However, this technique does not involve a method of forming a coil of a recent oxide high-temperature superconducting material nor does it involve a method of manufacturing a superconducting print coil optimal for a low magnetic field.
Another prior art system is disclosed in Japanese Patent Publication JP-A-62-140403. According to this Publication, leads of a superconducting coil used in a deflection electromagnet for an accelerator, etc., are crossed, a plurality of closed curved surfaces are provided in which reverse currents flow, and thus a uniform magnetic field is generated in the vicinity of the central axis of the superconducting coil without deteriorating the critical current characteristic of the elemental superconductors. The above conventional techniques are effective for use in a high magnetic field for accelerators, but the detection of a small magnetic field, for example, by a superconducting quantum interference element (hereinafter referred to also as a "SQUID") is influenced more adversely by external and/or internal noise than the uneven magnetic field generated by the coil parts.